1. Field of the Invention
This invention relates generally to an air-actuated brake system for vehicles and, more particularly, to improvements in such systems of the type which include separate primary and secondary sources of air under pressure that are so connected to brake assemblies as to apply either the service or park brakes even though air is lost from one of the sources.
2. Description of the Prior Art
U.S. Pat. Nos. 4,685,744 and 4,907,842, both assigned to the assignee of this application, show prior brake systems which, as is contemplated by this invention, may be used with various vehicle configurations such as powered vehicles or vehicles consisting of trailers towed by tractors whose compressors charge reservoirs of air on the trailer. Although U.S. Pat. No. 4,685,744 shows a preferred air actuated, mechanically held brake assembly, the present invention contemplates that the brake assembly may alternatively be a so-called spring brake as suggested in U.S. Pat. No. 4,907,842.
The prior system found in U.S. Pat. No. 4,907,842 includes a control means, sometimes called a "multi-function" or "MF" valve, that delivers air received from each of the sources directly to the brake chambers of associated brake assemblies. Thus, although the system was redundant in the sense that the service and park functions were split at the brake level, the loss of a hose leading to the brake chambers resulted in the loss of all parking function unless the operator of the vehicle applied it through the service side. In the case of a spring brake in a system of this type, a lost hose leading to a service brake would result in loss of the service function, and loss of a hose leading to a park brake would cause the brake to be applied automatically.
The object of this invention is to provide a brake system in which these and other problems in similar systems are overcome in that the service and park functions are split and mutually exclusive at all levels to provide redundancy throughout the entire system.